The present invention relates generally to protection circuitry for a power amplifier, and more particularly to an improved method and arrangement for maximizing the amount of output power from a radio frequency (RF) power amplifier which has a relatively small, or finite heatsink and which is operated intermittently over a period of time.
In general, for RF power amplifiers having transistors, 200.degree. C. represents a critical level of temperature for the devices. Moreover, for a radio operating in a mobile environment, it is often impractical to implement unlimited cooling for the RF power amplifier. In reality, most mobile radio applications operate with relatively little cooling due to the lack of circulating air and a relatively small, or finite-sized heatsink. As a result, this translates into a maximum heatsink temperature of 100.degree. C. in order to avoid seriously reducing the life of the devices. Therefore, the heat-dissipating devices within the RF power amplifier must be provided with some form of thermal protection to insure that the devices never exceed such critical temperatures.
Previously known power amplifiers typically have only one or two output power levels, with an elementary thermal protection circuit causing the RF power amplifier to switch to a lower power level upon sensing the temperature of various critical components (e.g., power output transistors) when the temperature of these components rises to a critical level.
In radios operating over a narrow band of frequencies, such thermal protection often merely takes the form of a thermal shutback switch or circuit, while in radios operating over a wide band of operating frequencies, previously known arrangements utilize both a temperature compensation circuit and a thermal shutback circuit. The temperature compensation circuit provides leveling of the output power for relatively large changes in frequency, while the thermal shutback circuit provides abrupt thermal shutback in the event of extreme operating conditions caused by high ambient temperatures or lengthy operation of the transmitter, as is encountered during continuous keyed operation.
However, such elementary forms of thermal protection for a power amplifier can create serious problems when the ambient temperature adjacent to the finite heatsink on which the power amplifier is mounted is already high, due to high surrounding air temperature or extremely long transmit (keyed) operating times. Such conditions may cause the transmitter to shut back to a fixed lower level that was preset to protect against every possible combination of worst-case operating conditions. During this shutback condition, the power output versus time characteristic may exhibit "ringing" as the temperature crosses over the threshold, and furthermore, the transmitter may produce so little output power during shutback that the quality of communications with desired receivers is jeopardized.
Accordingly, there exists a need for improved thermal protection for a power amplifier by remote sense which provides the combined advantages of thermal protection and power maximization, and which is able to ensure a safe operating temperature for the transmitter with a minimum of additional components while effecting gradual changes in the operating power level for varying environmental and operational conditions.